1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process, such as a laser beam printer, an LED printer, a facsimile, and a digital copying machine, and in particular, to a developing roller for an image forming apparatus employing a developing technique for forming a toner image using negative toner and a non-magnetic single component developing technique using a non-magnetic single component toner for developer.
2. Description of the Related Art
As illustrated in FIG. 1, an image forming apparatus using an electrophotographic process translates a digital image signal received from a computer or a scanner onto a recording sheet in the form of a visible image. The image forming apparatus includes a developing unit for forming a latent image according to the received digital image signal and attaching toner onto the latent image to form a toner image. Then, the toner image is fixed on the recording sheet, so that the original digital image signal may be printed out on the recording sheet in the form of a fixed image.
Referring to FIG. 1, the image forming apparatus uniformly electrically charges a surface of a photosensitive drum 1 by corona discharging of a charging roller 7. The digital signal received from the computer or scanner is converted into an optical signal by a laser scanning unit or an LED array 9. The optical signal forms an electrostatic latent image on the photosensitive drum 1. Toner 6 contained in a toner cartridge 8 is transferred toward a supply roller 3 by rotation of an agitator 4, and charged with a negative electric charge by the supply roller 3. Then, as a developing unit 2 rotates, the toner is transferred to a developing nip composed of the photosensitive drum 1 and the developing roller 2. A toner regulating blade 5 mounted on an upper part of the developing roller 2 forms a uniform toner layer on the surface of the developing roller 2. Thereafter, as the photosensitive drum 1 rotates, toner particles 6a charged with the negative electric charge are transferred onto the latent image, passing through a contact between the photosensitive drum 1 and the developing roller 2, thereby forming the visible toner image on the surface of the photosensitive drum 1.
Further, recording sheets 12 stacked in a sheet feeding cartridge 13 are picked up sheet by sheet by a sheet feeding roller 11 and transferred to registration rollers 14 and 14' placed at a front end of an aligning unit. Then, the recording sheet 12 is transferred to a transcribing nip which is a rotatable contact part between the photosensitive drum 1 and a transcribing roller (or a corona device) 10, in synchronism with a scanning unit 9. As the recording sheet 12 enters the transcribing nip, the toner image formed on the photosensitive drum 1 is transcribed onto the recording sheet 12 by an electrostatic force according to an air breakdown phenomenon by a high voltage applied to the transcribing roller 10, the high voltage having an opposite polarity to that of the toner. The recording sheet 12 on which the toner image is transcribed is transferred to a fixing unit by a transfer guide 15a, and the toner is fixed on the recording sheet 12 by the heat and pressure caused by a heat roller 16 and a pressure roller 17 in the fixing unit.
The image fixed on the recording sheet 12 is discharged to an exterior of the apparatus passing through lower sheet discharging rollers 19 and 19a, and upper sheet discharging rollers 18 and 18a, being guided by guide members 15b and 15c of a sheet discharging unit.
FIGS. 2A and 2B show a plane view and a side view of the image forming apparatus for use in an image printing device, respectively. In the conventional image forming apparatus shown in FIGS. 2A and 2B, the toner leaking out through transfer paths A and B may contaminate the photosensitive drum 1 in the shape of a belt, and the toner having passed the regulating blade 5 at an interval D where the toner, which has failed to undergo friction charging by a side wall of the developing roller 2 or the supply roller 3 and thus does not enough negative electric charge, may be transferred to the photosensitive drum 1. The non-image region keeps the negative potential. The contamination belt of toner may cause the contamination on both sides of the recording sheet 12.
Now, reference will be made to FIG. 3 to describe the paths A and B which are the origin of the above stated contaminations. As illustrated, toner 6, frictionally charged at a nip E formed between developing roller 2 and supply roller 3, is attached to developing roller 2 and transferred toward a lower part of regulating blade 5, as developing roller 2 rotates. Regulating blade 5 regulates toner 6 attached onto the surface of developing roller 2, so that a limited amount of the toner only may be transferred to the surface of developing roller 2 upon passing through regulating blade 5. Regulating blade 5 blocks transfer of the toner attached onto developing roller 2, so that a considerable pressure is caused at a space between the supply roller 3 and regulating blade 5 on developing roller 2. In order to prevent the contamination of recording sheet 12 and the interior of the apparatus due to the pressure, the conventional image forming apparatus shown in FIGS. 2A and 2B includes a circular seal (or rubber foam) 20a for preventing leakage of the toner among developing roller 2, regulating blade 5, and toner cartridge 8. In order to prevent leakage of the toner having passed through circular seal 20a, a side-wall seal 20b blocks leakage of the toner through side walls of developing roller 2 and regulating blade 5. Alternatively, any one of circular seal 20a and side-wall seal 20b is selectively used to prevent leakage of the toner.
FIG. 4 illustrates a principal part of an image forming apparatus disclosed in Japanese patent publication No.07-253715, filed by Tanaka, in which a low-friction material is coated over a contact part between a developing roller 22 and a regulating blade 23 disposed at an upper part of developing roller 22. Further, side-wall seals 24, comprising seal portions 24a and 24b, are mounted at both sides of developing roller 22 by using double-faced adhesive tape 24d. In order to prevent leakage of the toner through the path B, a seal piece 24c is mounted at both contacts of regulating blade 23 and side-wall seal portions 24a and 24b are mounted at both side walls of developing roller 22. It is noted that this prior art device does not include the circular seal 20a (FIGS. 2A and 2B) mounted at the supply roller.
By coating the contact part between developing roller 22 and regulating blade 23 by using the low-friction material, the prior art device described above may successfully regulate the toner layer, when developing roller 22 has no toner attached thereto (i.e., an initial condition before the toner is supplied), or when toner remains after use of the apparatus. Further, the device has side-wall seals 24 mounted on the side walls of the developing roller 22, so as to prevent leakage of the toner. However, from the viewpoint of sealing, if the supply roller does not have circular seal 20a, the toner in the developing unit has more opportunities to pass through the side wall, so that more toner may penetrate into a space between side-wall seal portion 24b and developing roller 22. As developing roller 22 rotates, the toner having penetrated between side-wall seal portion 24b and developing roller 22 is transferred to the photosensitive drum, causing the contamination belt of toner thereon. As a result, the recording sheet may be contaminated at both sides thereof.
Further, if an over-pressure is applied to the side walls in order to solve this problem, the side walls of developing roller 22 will be worn away so that the worn developing roller 22 may no longer maintain the toner sealing function. Accordingly, as shown in FIG. 5, the device may have an worn area 25 through which the toner leaks out.
FIG. 6 illustrates an image forming apparatus disclosed in U.S. Pat. No. 4,641,602, filed by Kasai, in which a high voltage having a polarity opposite to that of the toner is applied to a non-image region b of a developing roller 25, so that a regulating roller 26 may attract the unnecessary toner to prevent the unnecessary toner from being transferred to the photosensitive drum. However, the device proposed by Kasai has the following shortcomings in putting it to practical use.
First, U.S. Pat. No. 4,641,602 assumes that the toner is charged with only one of the negative and positive electric charges. Actually, however, the existing commercialized toner has a friction charge distribution feature as shown in FIG. 7. The toner maker makes a deep study to produce the toner which has a narrow charge distribution width and prevents generation of wrong signed toner particles having an insufficient charge A in case of necessity of the negative charge. Therefore, when using the commercialized toner having the charge distribution feature of FIG. 7, if the toner is charged with the negative charge, it may be impossible to electrically control the toner particles at an area A as expected.
That is, referring again to FIG. 6, in a developing technique using negative toner, in order to prevent the toner from being attached to the non-image region thereby to prevent leakage of toner, a negative voltage is applied to a non-image sleeve 25c of the developing roller 25 and a positive voltage is applied to a sleeve 26c of the regulating roller 26. The conventional device is effective for the toner at the areas other than the area A. However, the leakage of the toner may be increased undesirably at the area A. That is, the insufficiently charged toner, which is neutral or reversed, transfers excessively to the photosensitive drum, thereby causing the contaminations of the photosensitive drum and the recording sheet.
Second, the prior art apparatus can not prevent the toner from leaking through an interval "c" and the side walls of developing roller 25. This leakage becomes more serious when developing roller 25 pressingly contacts regulating roller 26 with a specified nip.
Third, insulating layers 25b and 26b at a boundary between an image region "a" and a non-image region "b" of the developing roller 25 and the regulating roller 26 cannot freely control the movement of the toner. Since the toner particles have a diameter of 3-15 .mu.m, the uncontrollable toner may leak out at this boundary little by little, thereby contaminating the photosensitive drum.
FIG. 8 illustrates an image forming apparatus disclosed in Japanese patent publication No. 04-085571, filed by Fukumoto, in which a PTFE (polytetrafluoroethylene resin) layer 29b is attached to a chloroprene rubber (or silicon rubber) 29a with hot melt or double-sided tapes 29c and 29d, in order to stably seal a developing roller 30, by using elasticity of the chloroprene or silicon rubber and a low-friction feature of the PTFE layer. However, the prior art apparatus is effective only for two-component or 1.5-component developing technique in which the toner is charged and transferred by a carrier which is much larger than the toner particle in size, and is placed on developing roller 30.
In the case of single component non-magnetic toner, the carrier (commonly having a diameter of over 3 .mu.m) is not used and the toner has a diameter of 3-15 .mu.m. Thus, it is impossible to prevent penetration of the toner between the PTFE sealing material and the developing roller. Particularly, in case of the old-fashioned polymer toner, it is more difficult to prevent leakage of the toner by the PTFE sealing material.
Furthermore, in the image forming apparatus proposed by Fukumoto, the toner penetrating between PTFE 29b and developing roller 30 is easily charged with a positive electric charge. That is, the principal ingredients of the toner is styrene and, in accordance with triboelectric charge service, the PTFE charges the toner with the positive electric charge more easily than nylon, polyurethane, SEP (Silicon Ethylene Propylene) rubber, and NBR (Nitril Butadiene Rubber) do. When using a known developing technique (e.g., a commercialized LBP or LED printer), unwanted reversely-charged toner (in this case, the toner charged with zero or "+" electric charge) exists on developing roller 30. In this case, since the toner has the polarity opposite to that of the non-image region of photosensitive drum 31, the toner may be readily transferred to photosensitive drum 31, thereby contaminating the photosensitive drum. Furthermore, in case of increasing pressure of the seal in order to prevent penetration of toner, both end parts of developing roller 30 which rub against PTFE 29b may be worn out.
As illustrated in FIG. 9, Japanese patent publication No. 03-102369, filed by Umezawa, discloses another image forming apparatus, in which a belt-shaped sealing material 33 is inserted between a photosensitive drum 31 and a developing roller 32 of which both ends are tapered. This prior art apparatus may effectively seal the space between the photosensitive drum 31 and the developing roller 32, but it needs a separate sealing plan in order to prevent undesired leakage of the toner from a developing chamber composed of the developing roller 32, a supply roller 34, and a regulating blade 35.